A network architecture called “IP Multimedia Subsystem” (IMS) has been developed by the 3rd Generation Partnership Project (3GPP) as an open standard for handling multimedia services and sessions in the packet domain (for details regarding IMS, please refer to 3GPP TS 23.228 found at http://www.3gpp.org/ftp/Specs/html-info/22228.htm). Currently, various communication terminals and devices (hereinafter referred to as IMS terminals or User Equipment (UE)) that conform to the IMS standard are known. A typical example of an IMS terminal is a mobile phone with IMS functionality. Personal computers (PCs), personal digital assistants (PDAs), and the like can also serve as IMS terminals if they are equipped with IMS functionality. IMS terminals can provide multimedia services by, for example, receiving video streaming from a video-streaming server over an IMS network.
Generally, UE has IMS subscription information such as an IMS Private User Identity (IMPI) and an IMS Public User Identity (IMPU). The UE can make access to a mobile IMS operator's network (hereinafter referred to as a mobile IMS network) via a mobile access network such as a General Packet Radio Service (GPRS). The mobile IMS network authenticates and authorizes the UE by use of IMS subscription information, and provides the UE with multimedia services such as Voice over IP (VoIP).
When the UE directly accesses the mobile IMS network and is provided with IMS services, the mobile IMS network can control the quality of service (QoS) over the mobile access network.
Meanwhile, UE equipped with a wireless Local Area Network (WLAN) adapter is also known. Such UE can join the LAN, which is connected to a fixed IMS operator's network (hereinafter referred to as a fixed IMS network) via a fixed access network such as a Digital Subscriber Line (DSL).
Because the IMS is an access-agnostic system, UE equipped with WLAN adapters may be provided with IMS services via the fixed IMS network under the condition that the UE can register with the fixed IMS network. The UE can register with the fixed IMS network when the mobile IMS operator of the UE has a roaming agreement with the fixed IMS operator, when the fixed IMS operator is the same as the mobile IMS operator of the UE (i.e., the IMS subscription information of the UE is also valid for the fixed IMS network), or the like. In these cases, the fixed IMS network can control QoS over the fixed access network when the UE accesses the fixed IMS network and is provided with IMS services.
However, a problem arises if no roaming agreement exists and the IMS subscription information of the UE is not valid for the fixed IMS network. In this case, the UE cannot be provided with IMS services via the fixed IMS network because the fixed IMS network rejects an initial IMS Registration request from the UE.
In some situations (e.g., in the case that an IMS Subscriber Identity Module (ISIM) application of the UE contains an address of the P-CSCF of the mobile IMS network), the UE can register with and access the mobile IMS network via the fixed access network using its IMS subscription information. In this case the UE can be provided with IMS services from the mobile IMS network via the fixed access network. However, QoS is not guaranteed because the mobile IMS network does not have privileges to control QoS over the fixed access network (see FIG. 1).